Recently, from a view point of the high energy price of crude oil or the like and the exhaustion of oil resource due to the increase in energy consumption volume on a global scale, there has been observed the vigorous energy source development with respect to oil fields having a large depth (deep layer oil fields) which had not been noticed, oil fields and gas fields in a severe corrosion environment which is so-called a sour environment containing hydrogen sulfide or the like, and oil fields and gas fields around the North Pole which is in a severe weather environment. Steel products used in these oil fields and gas fields are required to have high strength, excellent corrosion resistance (sour resistance) and excellent low-temperature toughness.
Conventionally, in oil fields and gas fields in an environment which contains carbon dioxide gas CO2, chloride ion Cl− and the like, as a steel product used for drilling, 13% Cr martensitic stainless steel has been popularly used. Recently, the use of improved 13Cr martensitic stainless steel having a chemical composition, wherein the content of C is decreased and the contents of Ni, No and the like are increased, has been spreading.
For example, patent document 1 discloses a method of manufacturing a martensitic stainless steel plate wherein the corrosion resistance of 13% Cr martensitic stainless steel (plate) is improved. The martensitic stainless steel plate disclosed in patent document 1 is manufactured by hot working a steel having a chemical composition containing by weight %, 10 to 15% Cr, 0.005 to 0.05% C, 4.0 to 9.0% Ni, 0.5 to 3% Cu, and 1.0 to 3% Mo, wherein the Ni equivalent amount is adjusted to −10 or more, followed by air-cooling to a room temperature, thereafter, heat treatment at a temperature which is equal to or above an Ac1 point at which an austenite fraction becomes 80%, and further, heat treatment at a temperature at which the austenite fraction becomes 60% or less. The thus manufactured martensitic stainless steel (plate) has a microstructure constituted of tempered martensitic phase, martensitic phase and retained austenitic phase, wherein the total fraction of tempered martensitic phase and martensitic phase is 60 to 90%. It is described in patent document 1 that the martensitic stainless steel (plate) enables corrosion resistance and sulfide stress corrosion cracking resistance in a wet carbon dioxide environment and a wet hydrogen sulfide environment to be improved.
Patent document 2 discloses a method of manufacturing a high-strength stainless steel pipe for oil wells having excellent corrosion resistance. The high-strength stainless steel pipe disclosed in patent document 2 is manufactured by heating a steel having a chemical composition containing by mass %, 0.005 to 0.05% C, 0.05 to 0.5% Si, 0.2 to 1.8% Mn, 0.03% or less P, 0.005% or less S, 15.5 to 18% Cr, 1.5 to 5% Ni, 1 to 3.5% Mo, 0.02 to 0.2% V, 0.01 to 0.15% N, 0.006% or less O, wherein Cr+0.65Ni+0.6Mo+0.55Cu−20C≥19.5 and Cr+Mo+0.3Si−43.5C−0.4Mn—Ni−0.3Cu−9N11.5 are satisfied, followed by hot working into a seamless steel pipe, cooling to a room temperature at a cooling rate equal to or above a cooling rate of air cooling, reheating to a temperature of 850° C. or more, cooling down to a temperature equal to or below 100° C. at a cooling rate of the air cooling or more and, thereafter, quenching-tempering treatment where the seamless steel pipe is heated to 700° C. or below. The high-strength stainless steel pipe has a microstructure containing a 10% to 60% of ferrite phase by a volume fraction and the balance being martensitic phase, and a yield strength of 654 MPa or more. It is described in patent document 2 that the high-strength stainless steel pipe for oil wells has high strength, sufficient corrosion resistance also in a high temperature severe corrosion environment up to a temperature of 230° C. containing CO2 and chloride ion Cl−, and further, high toughness with absorbed energy of 50J or more at a temperature of −40° C.